1. Field of the Invention
The present invention relates to a semiwide-screen television receiver and, more particularly, to a semiwide-screen television receiver whereby the effective horizontal picture size can be widened. Consequently, side portions of the broadcast picture, which are conventionally invisible due to overscanning, can be projected on the screen to provide a horizontally expanded picture.
2. Description of the Prior Art
Generally, television broadcasting stations transmit video signals having a 12:9 or a 4:3 screen aspect ratio (AR). Referring to FIG. 1, the overall number of scanning lines in the NTSC system is 525, while the number of scanning lines for the visible image is 485 (i.e., overall scanning lines minus scanning lines for the vertical blanking periods provides: 525H-2.times.20H=485H, where H is a period for a scanning line). Also, the effective horizontal scanning period is determined by subtracting the horizontal blanking period from one scanning period (1H)(63.5 .mu.s-10.9 .mu.s=52.6 .mu.s). As can be appreciated from the above, in a typical television receiver of the NTSC standard, less than 100% of the broadcast image is displayed on the cathode ray tube (CRT) screen.
FIG. 2 illustrates test patterns shown on the screen for testing the image received by the television receiver. FIG. 3 illustrates a test pattern, from the test patterns of FIG. 2, which is utilized for testing the overscanning.
In a typical NTSC type television receiver, an overscanning of about 9.4% in vertical and horizontal directions is performed. Therefore, vertical and horizontal edge portions of 9.4% of the 4:3 screen AR image signal transmitted from the broadcasting station cannot be viewed. Overscanning is performed in order to prevent displaying a distorted image on the edge portions of the screen due to an unstable deflection, unstable focusing, unstable high voltage, etc. of the CRT. Specifically, the inductance dispersion in the deflection unit causes instability of the image displayed on the edge portions of the screen.
That is, if the inductance of the horizontal deflection coil is greater than its specified value, the corresponding impedance is increased and horizontal deflection current is decreased. Consequently, the width of the image is narrowed and a part of the blanking interval may be viewed on the right and left edge portions of the screen. On the contrary, if the inductance is small, the corresponding impedance is decreased and horizontal deflection current is increased. Consequently, the width of the image is widened and a larger portion of the picture information is concealed at the edges of the screen. Overscanning helps avoiding these variations in the width of the displayed image.
In order to satisfy the limited conditions of the television receiver as described above, and to display a stable image on the screen, the overscanning of 9.4% in the vertical and horizontal directions has been adopted by manufacturers of television receivers since about 1960.
Recently, a wide-vision television receiver which displays a video signal having a 12:9 screen AR on a screen having a 16:9 screen AR has been introduced. According to this wide-vision standard, the scanning lines are not changed, but the image displayed on the screen is horizontally expanded by about 1.33 times. Meanwhile, techniques for receiving a 16:9 screen AR broadcast video signal and displaying it on a display screen of a 4:3 screen AR or of a 16:9 screen AR have been introduced. For instance, U.S. Pat. No. 5,386,236 discloses such a technique.
However, when a 4:3 screen AR broadcast signal is displayed on the 16:9 AR screen, the displayed image is distorted since the image is merely expanded in the horizontal direction to fit the screen. Further, when the received image is not expanded, black panels are shown on the top and bottom portions, or the right and left edge portions of the screen.